>Ok, lets think about the job responsibilities of the back-end in a modern-day program: it is responsible for making sure its data ok. Furthermore, it's NOT the job of the middle-tier to ensure that your data is valid: like I said, that's the job of whatever back-end you're using.

Travis,

It is the job of the database server to enforce the domain constraints on the data. It is the job of the business logic tier to enforce the business rules on the data. This is a point that is often misunderstood.

For example, a city field has a domain that allows and valid city name but the business rules say that only Canadian cities are allowed, what should the database use as a constraint and what should be enforced as a business rule. If the middle tier object is designed well it will much easier top change a rule at that level then it would to change the domain definition for the database when the client starts doing business in the US.

Business rules are expected to change over time, domain constraints do not chane.